The last statistic on phone screen sizes is the sweet spot is right around 4-4.5". Around 12% of people use phones larger then this size.
That's 12% of the population who isn't buying an Apple product.
Another problem is fragmentation which makes it harder for developers to make multiple Apps for different sized phones.
This shouldn't be an issue in this day and age when even our phones have enough CPU power and memory to use vector images without any performance issues. If iOS were built around maintaining a consistent size for icons and UI elements completely independent of resolution, then the problem of various screen sizes wouldn't be an problem at all. Of course there should be two UIs built to cater to larger screened devices vs. smaller ones, much as it is now for the iPhone and iPad. But it doesn't have to be so specific. The optimal goal should be to set up your OS to allow for a ton of flexibility among a variety of screen sizes without any sacrifice to usability.
And no, the current set up developers faces isn't because "Apple has determined the perfect size for the easiest to use UI among all their deviecs". It's either laziness on Apple's part, of they're having trouble converting over to a res independent UI. Case in point, the iPad Mini uses the iPad UI without any tweaks whatsoever. The icons haven't been resized, the various buttons and other elements are the same resolution as they are on the larger iPad...it's completely untouched. Yet despite this, I've heard no one complain about it being too hard to use because the UI wasn't "designed for an 8 inch screen".
You say you are waiting for Apple to do something great. Making multiple versions/screens is not great, just different.
Not everything has to be completely industry redefining and earth shattering before it should be implemented Larger screen sizes aren't innovative in the least, but they sure as hell are useful for the people who want or need them. So why is Apple waiting around to give us a larger iPhone?
Because it doesn't work? Obviously the market thinks otherwise. Even huge, hulking beast phones/tablet/phablet/wotzits like the Galaxy Note line are selling fairly well.
Because Apple doesn't want to pollute their product line up with too many options? The iPod and iPhone are the only two device Apple makes that doesn't have at least two size options.
Making hybrid laptop/tablet Apple is not going to do. All it would do is make a device that is neither a great laptop or a tablet. Touch on a laptop has always been sub par as well as a desktop experience on a tablet, not great as well.
I'm pretty sure hybrid laptops or tablets like the Surface Pro are the future. Like I've said previously, we're seeing an entire generation of kids growing up around touch based devices. They're going to want to use touch for everything computer related. Mice will soon be relegated to specialized tool status, and touchpads will be seen as a cheap alternative to touchscreens that does the job, but doesn't work nearly as well. It'll be a little while yet before it truly hits, but the change is coming.
And Apple knows it, I'm sure. They're currently doing what they always do, playing the waiting game until they come up with what they think is the perfect implementation of a hybrid device.